This invention generally relates to photovoltaic cells and, in particular, to a photovoltaic cell which includes a photoactive body sandwiched between first and second electrode layers of ionically conductive material, wherein the photoactive body is effective to generate electric energy in response to its exposure to a light source, while enabling the storage of energy in the cell which comprises a photovoltaic battery.
One of the significant problems encountered with solar cells is the storage of energy produced by these cells. It is desirable to have storage capabilities associated with solar cells since these cells are able to produce more energy than is needed at any particular time. Thus, a solar cell in a home heating application should be able to store the energy collected during the day for use overnight. Conventional silicon or gallium arsenide cells must be connected to storage batteries since they do not have intrinsic storage capabilities. Many examples utilizing this principle could be cited, one such example being shown on page 26 of the publication Machine Design, Vol. 46, No. 11, published May 2, 1974. The need for solar cells with higher power/weight and power/size ratios is especially acute in the areas of satellite communication and jet propulsion, since smaller and lighter cells are necessary for improved efficiency in launching and performance.
Another major problem concerning the efficiency of solar cell performance is the problem of recombination. In a typical silicon or gallium arsenide solar cell, a fraction of the photoelectrically produced minority carriers recombine before reaching the PN junction of the solar cell, thus significantly reducing the available power. Approaches to a solution of this problem including the introduction of a graded distribution of impurities into the P-region near the PN junction so as to induce an electric field which tends to force more electrons toward the PN junction. Other approaches center around modification of the PN junction itself, as exemplified by U.S. Pat. No. 3,682,708, issued to A. I. Bennett. None of these approaches have been totally successful, and devices that do significantly reduce the recombination rate are difficult to fabricate and expensive to manufacture.
Finally, a third major problem associated with conventional solar cells is the difficulty of fabrication. Typically, silicon or gallium arsenide cells must be grown over a long period of time in a highly controlled environment, and doping levels must be carefully monitored as the crystal is pulled from the base.